This invention pertains generally to carbon having a high surface area and particularly to monolithic carbon having a high microporosity and a high macropore volume resulting in high fluid permeability.
Porous carbon has been found to be useful in applications where high surface area (&gt;500 m.sup.2 /g) is particularly desirable, among these being: battery and supercapacitor electrodes; as media for separating and purifying liquids and gases and recovery and storage of gases. Conventional high surface area carbon is generally a particulate material having particle sizes in the range of about 15 .mu.m to 5 mm. For separations processes, this particulate carbon material is often packed into beds or columns through which a liquid or gas flows. The efficiency of particulate powder beds is reduced by the fact that particles cannot be packed to 100% of their theoretical density; there will always be some space between particles and the efficiency of the particulate bed is reduced by the interstitial spaces between the particles. Further, these spaces provide a means whereby the process fluid can flow through the bed without effectively contacting all the carbon particles. Fluids passed through columns or beds packed with carbon powder can become contaminated with small carbon particles eluted from the column or bed and entrained in the fluid.
When used as an electrode, particulate, porous carbon material is mixed with a binder and the mixture pressed into an appropriate shape. The internal resistance of carbon powder electrodes is dependent upon the extent and quality of particle-to-particle contact. As the quality and extent of these contacts decreases the internal resistance of the electrode increases which in turn degrades the performance of the electrode. Binders, generally being of higher resistance than the carbon particles they surround, will also increase the particle-to-particle resistance thereby degrading the performance of the electrodes.
It has been recognized that one way to overcome the problems associated with porous carbon powders is to develop carbon materials in the form of a continuous, monolithic structure and prepared in such a way so as to possess the desirable properties of high surface area and low electrical resistance. As illustrated in U.S. Pat. Nos. 5,260,855; 5,021,462; 5,208,003; 4,832,881; 4,806,290 and 4,775,655 carbon foams, aerogels and microcellular carbons have been developed which overcome many of the problems associated with porous carbon powders used for separations or electrodes, supra. Many of these carbons either have most of their porosity in the micropore (&lt;2 nm) or mesopore (2-50 nm) size range and, consequently, have low permeability to liquids and gases but have surface areas that are generally on the order of several hundred m.sup.2 /g. Other carbons possess significant macroporosity (0.1-1 .mu.m) and thus have increased permeability to liquids or gases but with surface areas that are typically very low (10-100 m.sup.2 /g). These carbons must be treated in some way in order to increase the surface area.
A composite, semipermeable membrane comprising a microporous adsorptive material supported on a porous substrate for use in separating multicomponent gas mixtures has been disclosed in U.S. Pat. No. 5,431,864. However, here, as in other methods of separating gaseous mixtures employing carbon, an activation step is required in order to increase permeability and selectivity of the carbon membrane.
In order to prepare high surface area carbons some sort of treatment is generally required to enhance the pore structure of the carbon. This step, generally termed activation, comprises heating the carbon in an oxidizing atmosphere such as air or steam. For many applications it is desirable that the porosity be tailored. In the case of adsorption of gases, it is known that pore size is a critical parameter of the adsorption process, i.e., the pore size should closely approximate the size of the gas molecule to be adsorbed. Thus, for greater adsorption of gases above their critical temperature the adsorbent should be microporous rather than macroporous. However, conventional methods of activating carbon are not pore-size specific. The use of oxidizing agents such as steam or heating the carbon in the presence of an oxidizing agent such as air increases both the macro as well as the microporosity of the carbon. The relationship between pore size distribution and the absorptive properties of carbons as well as a critique of activation methods is discussed in U.S. Pat. No. 5,071,820. As pointed out therein the activation process can require many cycles before the desired pore size distribution is achieved. Which, while the activation process is necessary for many applications, makes it very unattractive because of the long times required for the process to be accomplished.
As discussed earlier, a class of carbons, namely carbon foams, aerogels and microcellular carbons has been developed which overcomes many of the problems associated with carbon powders used for separations or electrodes processes. However, these carbons are typically derived from polyacrylonitrile (PAN) or PAN-based polymers and as such require a special processing scheme, namely a pretreatment or "preoxidation" step wherein the PAN polymer precursor material is carefully heated in an oxygen containing atmosphere, typically air, in order to stabilize the precursor material prior to the pyrolization step. Without this pretreatment or "preoxidation" step, carbonization of the PAN or PAN-based precursor material occurs with significant degradation of the polymeric material leading to low molecular weight fragments being formed in preference to carbon with a consequent low carbon yield. Because the pretreatment step is quite exothermic, failure to carry out the pretreatment step without careful control of processing conditions can lead to the PAN or PAN-based polymer precursor becoming so hot it may fuse, decompose or burn. While PAN or PAN-based polymers can produce monolithic carbon having desirable properties, the necessity for a carefully controlled pretreatment step prior to carbonization is a significant economic detriment to this method of preparing carbon for electrodes or separations processes.
For the reasons set forth above, there has been a particular need to develop a carbon material that has a monolithic structure, the high macropore volume associated with high permeability for fluids and a high surface area composed principally of microporosity, a tailored pore size distribution, and does not require either a carefully controlled pretreatment process in an oxidizing atmosphere to prepare the carbon material or an uncontrollable activation process to achieve high surface area. Responsive to these needs, a novel processing method has been developed for producing porous carbon materials for uses such as, but not limited to, electrodes for batteries and supercapacitors, for separating and purifying fluids and recovering and storing gases. Polymer precursor materials processed in accordance with the present invention can yield porous monolithic carbon materials which possess both high permeability (high macropore volume) and high surface area, wherein the high surface area is the result of microporosity (pores &lt;2 nm in diameter), without the use of an activation step.